The present invention relates to concrete structures, such as concrete support columns for bridges, and, more particularly, to reinforcement of such structures with a composite material.
Steel reinforced concrete structures, such as bridge supports and supports in parking structures, can occasionally experience forces beyond the forces for which they were designed. This has happened a number of times during earthquakes. In earthquakes, structures undergo an excessive strain for an extended period. This characteristic causes earthquakes to weaken structures until the structures fail. In an unconfined concrete column, the acceleration of the column caused by the forces of the earthquake cause the column to either be crushed or to be sheared and the outer portions of concrete to spall off. With this spalling off of concrete, the diameter of column is reduced, its ability to support an upper structure is decreased, and the column fails, along with the upper structure. The results have been catastrophic, with the collapse of bridges and other structures, loss of life and the loss of use of major highways for many months, and even years. The cost of rebuilding collapsed structures like bridges is so high that sometimes the structures are not rebuilt. Concrete bridge columns are typically 4 to 8 feet in diameter and 20 to 60 feet high. In an earthquake, the ground shifts not only laterally, but vertically. The lateral shift causes a failure at the column base or in the mid-column because of the inertia of the upper bridge structure being at rest while the lower structure shifts laterally. In the case of the 1994 San Fernando Valley earthquake, the ground also moved up from a thrust fault, which caused the columns to fail in the middle versus the lower sections, where they failed in the 1989 Loma Prieta earthquake.
Concrete columns have the additional problem that moisture penetrates the concrete and freezes, causing the concrete to spall off. The spalling increases with the number of freeze-thaw cycles.
Pre-1971 bridge columns in California had an insufficient amount of vertical and horizontal steel, with only 1/2" diameter circular lap-spliced reinforcing bars approximately every 12". In the 1971 Sylmar earthquake, the 1989 Loma Prieta earthquake, and the 1994 San Fernando Valley earthquake, many of these columns exploded because of the forces, either from the ground moving up in the earthquake or from the inertia of the bridge deck collapsing down. The columns exploded radially outward in a pear-shaped fashion. More-recently constructed columns have about twice as must steel comprising their vertical reinforcing bars and a complete circumferential reinforcement cage defined by a helical 3/4" diameter reinforcing bar having about 3 inches between adjacent turns of the helix.
Some concrete bridge columns which were already reinforced with embedded steel reinforcing bars have been retrofitted with steel jackets. The steel jackets typically have a thickness between 3/8" to 1 inch, depending upon a variety of conditions, including soil conditions, the original design of the column, the height of the column, the amount of load the column carries, etc. In order to retrofit existing columns with additional reinforcement, steel jackets made of semi-cylindrical sections are placed around the outside of the columns, and the sections and jackets are welded together to form adequate confinement. A drawback with the steel jackets is that they must fit as tightly as possible, even though the concrete columns are not always precise in diameter. In order to accomplish this, the columns are individually measured and those measurements are used to fabricate steel jackets of approximately the same diameter. The semi-cylindrical jacket sections are slightly oversized in radius, for example 1/2" to 1" oversized. After the jackets are welded in place, they are pumped with a pressurizing cement grout to serve as a medium to transfer from the concrete column to the steel jacket the loads imposed on the column. Sometimes a concrete slurry is injected between the steel jacket and the column, because of the difficulty in fitting the jacket to the column. However, there is shrinkage with the injected concrete and, therefore, there is inadequate load transfer between the column and the jacket. Furthermore, the steel jackets are very heavy and cumbersome to install, even with the aid of power cranes. Moreover, skilled workers, e.g., welders, are required to install the steel jackets, and the jackets are subject to corrosion. Thus, the steel jackets require maintenance. In addition, because the column may often be coated with a significant amount of residue and because the steel jacket may have rust on it, the bond between the two load transfer surfaces is often insignificant. Furthermore, the steel jackets make the column too stiff, which is a drawback for withstanding the forces of an earthquake.
The use of a resin pre-impregnated semi-cured material using carbon fibers or glass fibers or KEVLAR fibers and the use of a wet lay-up system involving high strength fibers and wet resin are currently being pursued. In the wrapping of columns with pre-impregnated tape, an entire machine must be brought to the job site. The use of the machine to wrap the columns can be very difficult in confining situations where the columns are placed very near walls.
Other support columns, which are commonly made of wood, such as utility poles, wharf pilings and bridge supports, occasionally experience exceptional forces, such as in winds or earthquakes. They also suffer from wood borers, other wood-eating pests, and general wear and tear. Furthermore, many wooden utility poles treated with creosote experience dry rot in their lower portions.